The invention relates to a method for controlling piezoelectric drives in filling level measuring devices.
Such a method is disclosed, for example, in German Patent Application DE 19 621 449 A1 in the name of the applicant. The design principle of a fork resonator is described there, and hereby incorporated by reference.
In reducing the size of tuning fork systems as far as very short fork prong lengths, the basic problem arises that in addition to the prong length the size of the remaining components must also be reduced to a corresponding extent, in order to obtain equivalent vibratory properties apart from the clearly higher frequency. While a corresponding reduction in the overall length of the piezoelectric drive unit is possible in principle, other components and parameters exist which do not change together in a suitable ratio.
Thus, for example, the diaphragm thickness is 1 mm for standard tuning forks with a prong length of approximately 100 mm. However, a diaphragm thickness of 0.4 mm would need to be targeted in the case of a tuning fork shortened to 40 mm. Since, however, a minimum material thickness of 1 mm is prescribed by statute for recognizing the diaphragm as explosion zone separation, the result is a crass disproportion between prong length and diaphragm thickness.
This problem is still further intensified in that the diameter of the central tension bolt on the diaphragm likewise cannot be reduced in size, since it would otherwise not be of sufficient tensile strength for the mechanical tensile stress applied by the drive system.
A weaker design of the drive system is, however, not possible since the diaphragm stiffness has increased owing to the constant diaphragm thickness in conjunction with a simultaneously reduced diameter. Since, in the case of a reduced diaphragm diameter, the tension bolt requires a larger area in relative terms, it leads to a further increase in the diaphragm flexural strength.
It is an undesired consequence of the diaphragm flexural strength, which is substantially too high by comparison with the fork prongs, that the fork prongs themselves take over a substantial portion of the overall flexural vibration of the vibration resonator. It is a particularly disturbing fact that, particularly in the case of a tuning fork covered by filling material, in addition to the fundamental vibrational mode vibration nodes also form on the fork prongs, with the undesired consequence of harmonic resonances.
In accordance with the prior art, a fundamental bandpass filter is fitted in the feedback oscillator which serves to excite the vibration resonator, and so the resonance circuit is reliably prevented from latching on to a harmonic. The partial formation of harmonic vibration nodes cannot, however, be excluded in this way, the result being a negative influence on the fundamental vibration.
The harmonic vibrations which occur have the effect that as the tuning fork dips into and out of the filling material the vibrational frequency changes not continuously but suddenly, with the formation of a hysteresis. In the case of viscous filling materials, it is even possible for the frequency profile to be inverted, since with increasing covering by filling material the influence of harmonic resonances grows. When the tuning fork dips into the filling material the frequency firstly dropsxe2x80x94as desiredxe2x80x94but with increasing cover there is then a rise in frequency under the influence of the high-frequency harmonics which, in the case of a completely covered tuning fork, can lead to a frequency value such as corresponds to an uncovered fork. If the fundamental bandpass filter is tuned lower, the problem arises that the fork resonator no longer starts to vibrate automatically when the power supply is switched on.
In the known solutions, a rectangular signal is used to control the piezoelectric element. However, since the rectangular signal has a very strong harmonic content in addition to the fundamental, harmonic resonances which are present, but undesired, in the fork resonator are excited.
The use of a harmonic-free sinusoidal excitation signal would certainly solve the problem theoretical, but in practice it is exceptionally complicated in terms of circuitry and very unfavourable in terms of energy. In addition to the power consumption of the sinusoidal generator, which can be controlled by frequency and phase in a variable fashion, sinusoidal output stages have a poor efficiency in principle and require a supply voltage which is increased by 2 so that a sinusoidal output signal of the same voltage-time area as a rectangular signal is generated. Furthermore, no method is known at present which permits the electronic separation of drive signal and detection signal in the case of sinusoidal excitation.
It is therefore the object of the method according to the invention to specify a method for controlling a piezoelectric drive in filling level measuring devices which, in conjunction with a minimum outlay on components and energy as well as the possibility of simultaneous use of a piezoelectric element for exciting and detecting vibrations, permits the fork resonator to be excited in a fashion attended by few harmonics.
This object is achieved by means of the features of claim 1. Developments of the invention are the subject matter of the dependent claims.
Thus, the method according to the invention achieves the object by virtue of the fact that an at least approximately trapezoidal signal is generated as excitation signal. The excitation signal can comprise, for example, two phases with approximately constant high or low potential which are interrupted by in each case a phase of defined period and a rate of signal change which is limited in a defined fashion. Use is preferably made for this purpose of a rail-to-rail integrator driven to the limit.